The present invention relates to hydraulic systems for driving the radiator cooling fans of vehicle engines, and more particularly, to an improved motor-valve apparatus for use in such fan drive systems.
Although it will become apparent from the subsequent description that the present invention has many uses and applications, it is especially advantageous when used to drive the radiator cooling fan of a vehicle engine, and will be described in connection therewith.
Originally, radiator cooling fans were driven directly, i.e., by some form of mechanical connection between the fan and the engine crankshaft. For example, the fan was frequently bolted to a flange on a shaft projecting forwardly from the engine water pump, such that the fan speed was either the same as the engine speed, or directly proportional thereto, depending upon the belt and pulley ratios between the crankshaft and the water pump. The proportionality between fan speed and engine speed is desirable at lower engine speeds (e.g., below 3000 rpm), but is undesirable at higher speeds where additional air flow through the radiator becomes unnecessary, wastes engine horsepower and creates excessive noise.
More recently, viscous fan drives have been developed which overcome the above-mentioned problems whereby direct fan drive systems have excessive fan speed at higher engine speeds. Viscous fan drives transmit torque by means of a viscous fluid contained within a shear space defined between an input member and an output member, such that rotation of the input member causes a viscous shear drag to be exerted on the output member, transmitting torque thereto. See. U.S. Pat. No. 2,948,268, assigned to the assignee of the present invention. Viscous fan drives of the type shown in the cited patent have an inherent torque-limiting characteristic, such that the fan speed increases roughly proportional to the engine speed, up to a certain engine speed such as 2500 rpm, then the fan speed levels off and remains constant as engine speed and torque continue to rise. The resulting graph of fan speed versus engine speed has become known as a "viscous curve", and it is now generally a requirement of U.S. vehicle manufacturers that any drive system, whether of the viscous type or not, operate in accordance with the well-known "viscous curve".
A further step in the development of viscous fan drives was represented by U.S. Pat. No. 3,055,473, which discloses a viscous fan drive having the same "viscous curve" during its normal operation condition (engaged), but in addition, has the ability to become disengaged in response to ambient air temperature being below a predetermined level, thus providing a substantial saving of engine horsepower when normal operation of the fan is unnecessary for sufficient cooling of the engine.
Both of the conventional fan drive arrangements discussed above can be used only with a standard in-line engine, i.e., one having the crankshaft oriented axially. However, in recent years many auto makers, especially in Europe, have elected to use a transverse engine, providing front wheel drive, for reasons which are now well known in the art, and the trend toward transverse engines, especially in the four and six cylinder range, is extending to the U.S. as well. In European transverse engine automobiles, the cooling requirements have generally been met rather easily for various reasons, including the fact that the majority of the vehicles have been used in the generally colder regions, such as the Scandinavian countries. However, the nature of the U.S. automobile market is such that all transverse engine vehicles marketed in the U.S. will be required to have sufficient cooling capacity to operate satisfactorily under the conditions prevailing in the hot southern regions of the country.
A major approach to the cooling of transverse engines is the use of hydraulic systems, including a hydraulic pump driven by the engine and a hydraulic motor connected to the fan. It will be appreciated by those skilled in the art of automotive engines and engine accessories that the addition of a complete hydraulic system creates problems relating to space requirements, and undesirably increases both the weight and cost of the vehicle. Accordingly, those attempting to design a satisfactory hydraulic fan drive system have tried to reduce the space, weight, and cost of such systems by utilizing at least one of the hydraulic components in at least two different vehicle hydraulic systems. For example, there have been frequent attempts to utilize the power steering pump to provide pressurized fluid to operate a hydraulic fan motor, as well as the power steering gear (see U.S. Pat. No. 2,777,287). In such systems, inter-action between the fan motor and the other hydraulic actuator (such as the power steering gear) have generally resulted in unsatisfactory performance by the fan motor, or the steering gear, or both.
One design approach to such systems has been to place the fan motor in series with the power steering gear, but upstream therefrom, such that the flow through the fan motor also passes through the steering gear. See U.S. Pat. No. 3,659,567. A major drawback of such prior art systems has been a constant flow rate through the fan motor over all engine speeds from idle to maximum, such that fan speed is constant regardless of engine speed. Typically, the result with such a system is that more cooling than is needed is provided at lower engine speeds, thereby wasting engine horsepower, or the cooling may be only marginal at higher engine speeds, or both. In addition, such systems require a relatively high fan motor pressure and pump horsepower at lower engine speeds when the pressure drop across the power steering gear is greatest, thus making it difficult to satisfy the pressure and flow requirement of both the fan and steering system simultaneously.
In parent application Ser. No. 907,064, the embodiment of the motor-valve apparatus illustrated therein includes a temperature responsive bypass valve which might be termed "mechanical position sensitive". By that it is meant that the operating characteristics of the bypass valve are largely determined by the setting of the mechanical connection between the bypass valve and the temperature responsive device, i.e., the power pill. Partially as a result of the mechanical connection to the temperature responsive device, a preliminary adjustment of the position of the bypass valve is required, and although the bypass valve may be set to operate satisfactorily at a given flow rate, it may not be fully satisfactory at other flow rates.